Exit code should not be successful on restructuredtext errors

Currently, if there is a restructured syntax error, the exit code is still 0.

    $ make publish
     ...
     .../content/article.rst:34: (ERROR/3) Unexpected indentation.
     .../content/article.rst:36: (WARNING/2) Block quote ends without a blank line; unexpected unindent.
     Done: Processed 60 articles and 1 pages in 3.03 seconds.
    
    $ echo $?
    0

I [use Travis to build the blog](http://blog.mathieu-leplatre.info/publish-your-pelican-blog-on-github-pages-via-travis-ci.html). It would be great if I could publish the output only if there is no rST error at all :)

    # .travis.yml
    script:
       - make publish
    after_sucess:
       - make github

This change allows to fail and exit loudly.

     $ make publish
     ...
     .../content/article.rst:34: (ERROR/3) Unexpected indentation.
     .../content/article.rst:36: (WARNING/2) Block quote ends without a blank line; unexpected unindent.
     make: *** [publish] Erreur 13
    
    $ echo $?
    2